BEHAVIOUR OF COAL ASH AS STRUCTURAL FILL MATERIAL

Abstract

Utilization of coal ash in geotechnical applications such as filling of low lying construction sites, embankments, dykes etc. is the most promising solution to the problem of disposal of coal ash in large quantities. In endeavoring to design and construct such structural fills, it is important to recognize the peculiar properties of coal ash and their implication on the performance of fills. The present investigation is an attempt to study the behaviour of coal ash (Class F ash) obtained from a thermal power plant with respect to compaction properties, collapse, effect of leaching on geotechnical properties and load-settlement behaviour. The performance of fills depends on compaction or densification of the material used in fills. The maximum dry density (MDD) and optimum moisture content (OMC) obtained by Proctor (standard / modified) compaction test are the benchmark parameters for the determination of the quality of compaction. Literature indicates that use of fresh / remolded sample for each compaction and the time allowed between wetting and compaction (preconditioning period) have significant effect on the values of compaction parameters (Johnson and Sallberg, 1962, Sivapullaiah et al., 1998). Compacted fills may be subjected to inundation due to rise of ground water level, infiltration due to rainfall etc. Literature shows that nearly all types of compacted soil fills experience collapse and the collapse potential is affected by initial dry density, compaction moisture content, inundation stress, clay content etc. (Chandra and Garcia, 1984, Lawton et al., 1992, Rollins et al., 1994). Therefore, (iii) adequate knowledge base on factors affecting collapse potential of compacted coal ash will help in design and construction of ash fills. Structural ash fills may be subjected to flow of water through them due to infiltration of rainwater or otherwise during their service life, leading to leaching of the fill material. Leaching may affect the geotechnical properties of fills as in the case of soils having soluble bonding elements (AlNouri and Saleam, 1994, Ismael and Mollah, 1998) and consequently the performance of structures placed on them. Compacted coal ash fills are often used as construction sites, and the bearing capacity and settlement are the required input for the design of foundations resting on such fills. Fly ash and bottom ash are cohesionless and it is logical to presume that methods of estimating settlement adopted for sand (i.e. cohesionless soil) are applicable in the case of ash fills also. However, compacted coal ash exhibits capillary stress due to contact moisture and preconsolidation stress due to compaction process (Seals et al., 1977, Toth et al., 1988) and it is reported that settlement of footing estimated using the methods based on plate load test and penetration tests as applied to sand results in considerable overestimation (Leonards and Bailey, 1982). Based on a detailed review of literature pertaining to the above aspects, the present study has been directed towards the following objectives: 1. To study the effect of (i) use of fresh / remolded samples and (ii) different preconditioning period on maximum dry density and optimum moisture content and suggest a standard procedure for preparation of sample for Proctor test on coal ash. 2. To study the effect of various factors influencing collapse potential of compacted coal ash. (iv) 3. Tostudy the effect of leaching on geotechnical properties of coal ash. 4. To study the load-settlement behavior of compacted coal ash and suggest a method for the estimation of settlement of footing resting on compacted ash taking into account the capillary stress and preconsolidation stress. COMPACTION PROPERTIES OF COAL ASH Fly ash and bottom ash (Class F ash), procured from thermal power plant at Dadri, 50 km East of New Delhi, were used as test material. The fly ash was collected from hopper and bottom ash from ash mound. Proctor compaction tests were carried out keeping (i) use of fresh / remolded samples, (ii) preconditioning period and (iii) compaction energy as variable parameters. The reproducibility of results of Proctor test and the effect of using fresh / remolded samples for each compaction and preconditioning period on compaction parameters are brought out. COLLAPSE POTENTIAL OF COAL ASH The collapse potential of fly ash (FA), bottom ash (BA) and mixture of fly ash and bottom ash in equal proportion (FA50+BA50) was determined by single oedometer collapse test, double oedometer collapse test and plate load collapse test. The specimens for oedometer tests were prepared by static compaction method using a special mould fabricated for the purpose. The tests were conducted with compaction moisture content, relative compaction, inundation stress, compaction energy and aging period as variable parameters. Plate load collapse tests were conducted on 120 mm plate resting on test bed prepared in a cylindrical tank of 590 mm diameter. The tank is provided with arrangements to effect rise of water level uniformly over the entire area of the test bed. The test plate was loaded to a predetermined loading (v) pressure and the settlement recorded for various positions of water level below the plate. The oedometer collapse studies show that compacted fly ash is susceptible to significant collapse while bottom ash and the mixture of fly ash and bottom ash in equal proportion show negligible collapse on inundation. However, when a test plate on a compacted bed of FA50+BA50 is loaded and the bed inundated, the plate undergoes significant amount of settlement mainly due to shear displacement in the bed immediately below the plate. EFFECT OF LEACHING ON GEOTECHNICAL PROPERTIES OF COAL ASH The effect of leaching on geotechnical properties of a mixture of fly ash and bottom ash in equal proportion has been studied by two approaches. In one, dry ash was leached and in the other case, ash was compacted and then leached in a set up allowing continuous flow of water. The un-leached and leached samples were tested for compaction and shear strength properties. Consolidated drained direct shear tests were carried out on specimens at compaction moisture condition and saturated condition. The results of these studies show negligible change in compaction parameters, MDD and OMC and a marginal increase or no change in strength parameters, c' and <)>', due to leaching. SETTLEMENT OF FOOTING / PLATE ON COMPACTED ASH FILLS An analytical method has been developed to estimate the load-settlement behavior of plate / footing resting on compacted ash fills taking into account the effect of capillary and preconsolidation stresses. The non-linearity of load-settlement (vi) behaviour has been appropriately modeled based on available plate load test data and incorporated in the method. The method requires as input, capillary stress, preconsolidation stress, and Young's modulus of compacted ash. Test procedures to evaluate these have been outlined. The proposed method has been validated by conducting plate load tests on laboratory prepared compacted ash beds and comparing the observed and predicted load-settlement behaviour. CONCLUSIONS Based on the above studies, the following major conclusions have been drawn: 1) Compaction properties such as maximum dry density and optimum moisture content of class F ash by Proctor test shall be determined by using fresh sample for each compaction and allowing a preconditioning period equal to the expected time interval between wetting and compaction in the field. The maximum dry density and optimum moisture content shall be obtained as the average of atleast three replicate tests. 2) Compacted fly ash is observed to be susceptible to significant collapse under different conditions of inundation stress, moisture content and relative compaction while compacted bottom ash and mixture of fly ash and bottom ash in equal proportion show negligible collapse under similar conditions. 3) Collapse potential of compacted fly ash increases with decrease in moisture content, decrease in relative compaction and increase in inundation stress. 4) Collapse settlement observed in plate load test is more than the one computed using collapse potential obtained from oedometer collapse test. The settlement estimated using collapse potential is only that component (vii) ofsettlement of footing / test plate which is due to compression of the ash bed on inundation and not the total settlement which is due to both compression and shear in the loaded bed. 5) Leaching has no measurable effect on grain size distribution, specific gravity, maximum dry density, optimum moisture content and shear strength parameters ofthe coal ash studied in the present investigation. 6) Amethod has been suggested for the estimation ofsettlement offooting resting on compacted ash taking into account the capillary stress and preconsolidation stress. The load-settlement behaviour predicted using the method is found to match fairly well with that observed in small-scale plate load tests. (